RCA splitting: Why is it able to send full output to 2 devices?

[jaytrinitron]

So this is a basic and very dumb question, but I am currently splitting my DAC line out RCA to a set of 2.1 speakers (Corsair SP2500, old and poor quality I know) and my headphone amp. It is my thinking that, should either the speakers or the amp be the only device on, the full output of the line out will reach whatever device is on. If both the speakers and the amp are on, then the volume would be reduced since the output is getting split.
In practice however, I notice no difference between having both my speakers and my headphone amp on as opposed to only having one of them on; the volume appears to be same to my ear. Reading more about it online, it appears that splitting RCA line outs like this is a clean way to achieve line out audio to 2 devices, but I don't understand why. To me it seems that there should be some loss of signal if two devices that both accept a line level signal are plugged in when the DAC is only rated to output 1 full line level signal. Am I right and is the output quality reduced in some way that is not perceptible in terms of volume, or am I wrong and the signal is the same to the speakers and headphone amp?

 

[hyperplanar]

If the input impedances of the speakers and the headphone amp are both high (let's say 10k ohms) then it may work in practical terms. What happens is that your DAC's line out basically sees a 5k ohm load on the other side since there are 2 10k ohm loads connected (so the output current would double, but since the impedance is still high, it's a miniscule amount) and the voltage it puts out can stay the same. Not too sure on how the devices would interact with each other when connected in such a way though.

 

[Bob-23]

You'd have a loss in voltage if your Y-cable were splitting the dac's output in series, but as it splits the signal in parallel, voltage remains the same. It's the same in your house: all your sockets are connected in parallel, thus voltage on all of them is equal. (Current may finally be insufficient if too many devices are plugged in.)

 

[BDWoody]

. It's the same in your house: all your sockets are connected in parallel, thus voltage on all of them is equal.

Got there before I did.

If both the speakers and the amp are on, then the volume would be reduced since the output is getting split.

When you plug in both your microwave and your fridge into the same circuit, they both get what they need, assuming your circuit isn't overloaded.

 

[Speedskater]

However there are (or were) some hi-fi components that would do nasty things when their power was off. In effect the input would have a diode across the input.

 

[MrPeabody]

As others have noted, when multiple loads are connected in parallel, the voltage that each sees will be the same as the voltage that one of the loads would see if it were the only load, assuming that the device supplying the voltage and the associated current doesn't object to delivering the additional current. In general, if a device supplying voltage and the associated current cannot tolerate the amount of current it is forced to supply to all the loads connected in parallel, something undesired might happen. The undesired thing that happens can take on several different forms, the worst of which is a fire, but no matter what does or doesn't happen, the current delivered to a specific load will not be lowered or limited except in conjunction with the lowering or limiting of the output voltage.

When the device supplying voltage and current is an audio amplifier, the amplifier may shut down if the current it is compelled to supply is in excess of its capability. In specific cases there may be circuitry that reacts to an excessively high level of current by reducing the amplifier's gain, which will lower the output voltage and thereby lower the current.

In the case of the line-level outputs, these are intended to feed high-impedance inputs which draw only a very slight amount of current. As such you can usually stack multiple Y-connectors as needed and there will be no drop in voltage and each connected device will receive all the current it wants to suck out of the preamplifier or receiver. Obviously there has to be a limit, i.e., a maximum amount of current that will be tolerated. I don't recall ever seeing this stated in the specifications for a preamp or receiver with preamp outputs, and I suspect the reason is that in practice it is rarely if ever a problem. As long as you don't stack Y-connectors to the point where it looks like what Chevy Chase did in the movie "Christmas Vacation", you aren't likely to have any problem.

 

[jaytrinitron]

Thanks all, these explanations make sense. I guess I should have just thought back to circuits class many years ago! Yeah, since it's parallel I guess my DAC just has to deliver (slightly) higher current if both devices are on, but the input impedances are so high it's not a problem for the DAC to deliver slightly higher current. Makes sense. In any case, I usually run with only 1 device on anyway, and in that case barring any strange behavior I think it should act the same as if it was plugged in directly without a splitter.

However there are (or were) some hi-fi components that would do nasty things when their power was off. In effect the input would have a diode across the input.

I assume if something like this happened it would be detectable, so I think this isn't the case with my equipment.